The present invention relates to hydraulic power steering devices, for example, for use in motor vehicles.
Motor vehicles, especially trucks and like large vehicles, are equipped with a hydraulic power steering device for assisting the driver in the steering force. Such power steering devices generally comprise a cylinder, and a piston slidably fitted in the cylinder. A hyraulic control valve is provided between an input shaft and an output shaft connected thereto by a torsion bar and operates to move the piston and produce a steering force. In the case of hydraulic power steering devices of the ball screw type, for example, the piston is integral with a ball nut having a rack in mesh with a sector gear on a pitman shaft, which in turn is coupled to the front wheels via a pitman arm, linkages and knuckle arms.
When the steering wheel is turned leftward or rightward to the greatest extent to a limit position in the case of such hydraulic power steering devices, the piston is left subjected to the hydraulic steering force. The hydraulic steering force is great especially in the case of trucks or like large vehicles, so that the possible break in the linkage becomes a problem when the steering wheel is turned to the greatest extent. With hydraulic power steering devices of the ball screw type, the pressure oil supplied from the pump for maximum steering operates the steering device with a pump relief pressure maintained, and the resulting force is likely to apply an excessive load to the knuckle arm stopper via the pitman arm and the linkage. Consequently, the device has the drawback of unexpectedly causing the linkage or knuckle arm to develop trouble. The linkage, knuckle arm and like components must therefore be given higher strength than is needed.
In the case of hydraulic power steering devices of the rack-and-pinion type, the piston is fixed to a rack bar, which is coupled to the wheel via a linkage and a knucle arm. The same problem is encountered also in this case.
Accordingly, a hydraulic power steering device has been proposed which comprises a cylinder and a piston having a piston ring attached to the outer periphery thereof and in which the cylinder is formed in its inner periphery with a recess having a width larger than the width of fit of the piston to the cylinder and opposed to the piston ring when the piston reaches a position in the vicinity of each of its opposite stroke ends. The oil chambers on opposite sides of the piston within the cylinder are adapted to communicate with each other through the recess in the inner periphery of the cylinder upon the piston reaching the position in the vicinity of the stroke end (see Unexamined Japanese Utility Model Publication SHO 60-122277).
With this device, the oil chambers on opposite sides of the piston within the cylinder communicate with each other through the recess upon the piston reaching the position close to the stroke end, thereby permitting the pressure oil to escape from the chamber of high pressure into the other chamber of low pressure to prevent the pressure from rising beyond a certain level and consequently precluding the linkage from breaking.
However, the piston ring of the device is usually made of synthetic resin, is subjected to oil pressure on one of its end faces facing the chamber of high pressure and on its inner periphery and is therefore deformed radially outward when brought into register with the recess in the cylinder inner periphery upon the piston reaching the position close to the stroke end. The piston ring is then in contact with the piston over a greatly diminished area of the grooved wall surface of the piston. In this case, another problem arises in that the piston ring will be damaged or broken, failing to seal off the pressure oil.
Another power steering device is also proposed which comprises a piston or cylinder having a relief valve that functions to release oil of high pressure when the piston is brought close to each of its opposite stroke ends (see Examined Japanese Patent Publication SHO 52-45093).
Nevertheless, the relief valve provided for the piston or cylinder renders the device complex in construction and costly.